Ergative Verbs and some discussion about them

In order to understand what is meant by 'ergative verb' in English, it may be important to understand the concept of ergativity in the first place. Ergative (or ergative case) generally refers to the marking of agent in transitive constructions while the agent in in-transitive constructions is not marked or is marked in the same way as the patient of a transitive construction (this is called absolutive). Languages where the ergative construction dominates are called 'ergative languages' (although there are no pure languages and there is great variation in the alignment of intransitive Subject, transitive Subject [called Agent] and Object [sometimes called Patient] across languages).

This is in contrast to the accusative languages where the dominant way of marking the X Yed Z construction is like in the English: 'She killed him.' (Xnom Yed Zacc).

In this context, it makes no sense to talk about ergative verbs. Just ergative constructions or ergative case marking on the Agent (and/or absolutive on the Patient).

The ergative verb in English refers to verbs that are normally transitive but that can be used without specifying the object because the object is also the subject.

So compare the verb open when use as in

1. John opened the window. / John baked the cake.
2. John opened for Mary. / John baked all morning.
3. The window opened. / The cake baked in 20 minutes.
4. ?The windows opened itself. / ?The caked baked itself.

Only the examples in 3 are ergative because the unexpressed object (patient) and subject (agent) are the same.

An easy test of this is whether the sentence can be passivised with the indirect object marking the agent.

1. The window was opened by John.
2. (The door/it/something) was opened by John for Mary.
3. *The window was opened. / *The window opened by John.
4. The window was opened by itself. (in some sort of cartoon universe)

To answer your questions directly:

1. All English ergative verbs are transitive (although some may also have intransitive variants). Ergativity is about expressions of transitivity.

2. The decision on whether a verb can be used as ergative is largely semantic. Can the agent and the patient be the same and are they both expressed in the subject position? So for example, 'cut' or 'kicked' cannot be ergative because if you take away the subject you end up with either an ellipsis or an intransitive verb. E.g. The knife cut deep. or The mule kicks. You could imagine some metaphorical meanings to include ergativity such as 'The engine kicked into gear.' But it's not clear in this case.

3. The decision about the subject is also semantic. Can the subject be an agent and patient at the same time? Contrast: 'Mary baked in the kitchen all afternoon.' vs. 'Mary baked in the sun all afternoon.' The first is intransitive use of bake and the second is ergative.